This invention relates in a general sense to methods and apparatus used in the handling, i.e. transmittal, processing, and utilization, of streaming digital data, such as are present for example in data storage and retrieval carried on as an adjunct to computer operations generally, as well as in communications and numerous other such activities. In a more particular sense, the invention pertains to novel methods and apparatus for in effect reformulating such digital data streams for the purpose of removing irregularities and variations which occur both randomly and otherwise in the timing and sequence of the bits (recorded bursts) of which such digital data is comprised, to thereby eliminate a substantial and potentially unacceptable source of processing error.
While the invention has broad aspects and applications, the nature of the problem apprehended and resolved, as well as the solutions provided, may be illustrated by reference to a particular example, involving random and varying degrees of data-burst timing change or displacement, which is becoming increasingly prevalent as bit density increases in data-processing and storage applications, a condition seemingly occurring on a continuous basis in the progress of the technology generally. Thus, while in a broader sense the invention provides technique for removing extraneous information from digital data streams generally, a particular example of the underlying problem is found in those numerous computer applications in which digital data is routed to and from the computer by use of peripheral storage devices such as a "floppy disk" drive and/or a tape drive, in particular streaming tape drives which use magnetic tape in cartridge form, which are essentially interchangeable with the floppy disk drive, fitting into the same standard-sized physical space and utilizing a standard floppy disk drive controller.
In a system such as that just noted, the "read data" stream is outputted to a data separator contained within the computer, although a data separator may also be built into the tape drive itself. In either event, the read data stream communicated between the tape drive and the computer is subject to the characteristics of the computer data separator as well as the characteristics of the tape drive and the tape cartridge. One aspect of this situation is the fact that practically all tape-handling apparatus, e.g. tape drives and tape cartridges, directly affect the data stream written onto and read from the tape itself, as a result of numerous mechanical and magnetic characteristics of both the drive and the cartridge, tape-handling being a physical process involving motion and contact with moving recording tape.
Thus, while the digital data stream initially provided from the computer may have substantially no significant deviations or other such errors in the timing of the data bursts, the tape drive and/or tape cartridge is very likely to produce timing changes in the sequence of data bits, particularly when the data is "interchanged", i.e. recorded on a tape cartridge by use of a first drive and then subsequently reproduced from that cartridge by use of a different tape drive. At the same time, practically all tape cartridges are known to exhibit speed variation characteristics when the tape is transported from one end to the other, and back and forth. Moreover, such speed variations may occur somewhat randomly as a result of such things as longitudinal flexure of the recording tape and of the drive belts used within the tape cartridges, particularly in what is perhaps the most prevalent such cartridge in current use, which utilizes drive belts having a significant amount of elasticity.
As a result of these effects, together with physical contact within the cartridge between the tape and various tape guides, etc., tape speed variation occurs randomly and in varying magnitudes, including "instantaneous" speed variation, known as "ISV". Such tape speed variations (in particular, ISV) have an understandable effect on the timing of data "written" on and "read" from the tape, acceleration resulting in the effective decrease of both the spacing between recorded bits and the duration or width of the recorded bits themselves, while decelerative speed effects cause increases in such spacing and duration. The net result appears as a frequency modulation of the resulting data stream, of a randomly-occurring nature and varying extent, but which is likely to have certain basic characteristics for a given tape cartridge caused by the particular physical aspects of that cartridge.
Depending upon a variety of factors, including the data separator utilized in a given computer application, the aforementioned modulation of the digital data stream may well have such an extreme effect on the timing of the bits in the data stream as to place their occurrence outside the allowable "window" (i.e. time slot) during which the presence or absence of the data bit has logical significance, thereby resulting in extensive bit rate error. Particularly where, as frequently occurs, the tape drive is utilized in conjunction with a floppy disk drive, and especially where it actually utilizes a floppy disk controller, the data separator in the computer may well have been optimized for the conditions characteristic of the floppy disk drive, since that is more likely to be the basic operating drive which is present in all applications (the tape drive being primarily for archival or "back-up" purposes). However, disk drives, in particular floppy disk drives, have much different characteristics than tape drives and are subject to much different and substantially less ISV characteristics. Therefore, in a given application, the computer data separator may not be able to consistently track the digital data stream from the tape drive, at least where the tape cartridge used is one having characteristically high, or substantial, amounts of ISV, which produce substantial frequency modulation effects in a resulting digital data stream.
As indicated above, the particular example described is merely that of a characteristic and recognizable situation actually present in state-of-the-art, commercially-available equipment in which data stream timing irregularities are caused by the tape cartridge; in fact, however, such data stream irregularities may result from a number of causes and be present in a number of circumstances, in data communication as well as in data storage and retrieval. Consequently, the novel solutions provided by the invention should be understood in this broader sense, even though illustrated for clarity and convenience by reference to a particular situation involving tape drives and tape cartridges.